1. Field of the Invention
The invention applies to sensor systems where the presence or absence of an object within the scan area must be established and more particularly where the presence or absence of an object within the scan area must be determined where many objects are present and many other movable sources of scan energy are also present in the general area leading to the production of spurious signals which interfere with the operation of the sensor system.
2. Description of the Prior Art
With the modern trend away from fixed assembly lines where objects in process are moved from one work station to another fixedly arranged along an inflexible conveyor line to flexible assembly, the work stations are now placed on the factory floor, as desired, and the objects in process are moved randomly from work station to work station depending upon the product being manufactured. The flexible path object which replaces the fixed conveyor line or belt is often the automatic guided vehicle or AGV. The AGV is free to move along the factory floor and present its object in process to each work station according to its control program which program can be changed at will each time production is shifted to a new product. Since the travel paths of the AGVs may cross one another, it is essential that the AGVs do not collide with each other or with another object or person. A commonly-employed technique is to operate the AGV at some prescribed speed and in the presence of an object to slow the AGV to creep speed and sound an audible warning signal and finally to stop if the AGV's bumper engages a solid object.
A common sensor system employed with AGVs to locate objects in their paths is the ultrasonic sensor system which transmits a burst of ultrasonic energy along a path in front of it and receives the return signal from any object in its path. If a single AGV is operating along a prescribed path and the transmitted energy is narrowly contained, the absence of any return or the absence of a return within a prescribed time period indicates that there is no object in the path of the AGV and it may advance at its regular speed. If a return within the prescribed time period occurs the AGV slows to creep speed until an object is encountered.
The presence of two or more AGVs operating in a confined area complicates the functioning of the sensor system. Two AGVs each having a sensor system operating at the same frequency of ultrasonic transmission can read each other's transmissions and misinterpret them as returns from an object in their path. Ghost returns, which are returns from objects beyond the normal scan area of the sensor system but which return in a proper time orientation with respect to any transmitted energy burst can be read as objects in the AGV's path, etc. In response to all of these false or spurious returns, the AGVs slow to creep speed and sound their alarms and continue to do so until no return is received.
Proposals have been made to reduce or eliminate this interference between AGVs working in confined areas using ultrasonic sensor systems but each have their drawbacks and limitations and usually result in severely limiting the number of AGVs which can operate at one time. One solution is to operate each sensor at a different frequency. While this might be practical with just a few sensors in a given location, it becomes very difficult to manage if many sensors are involved due to the limited range of operating frequency of most practical transducers and the minimum practical bandwidth of the receiver used with each sensor. Another is to assign different amplitudes to the transmitters of each sensor system but this is not practical since the received echo or return signal amplitude bears no traceable resemblance to the burst amplitude transmitted. Accordingly, modulation of burst amplitude to produce a burst signature to identify a particular transmitter is not practical either.
The use of the burst duration to identify a particular transmitter and thus a sensor system is not useful since the duration of the echo can be altered by the type or quality of the target struck. For example, the echo duration would be longer if it were to strike a set of stairs than a flat wall, although both objects were essentially at the same distance from the transmitter.